The present invention relates to the use of induction heating systems, more particularly, to the use of smart susceptors to selectively heat a part or parts during a manufacturing process.
Generally, induction heating processes may be carried out using any material that is electrically conductive and that generates heat when exposed to an electromagnetic flux field. Often, induction heating is used to directly heat an electrically conductive part during a manufacturing process. The electromagnetic flux field can be generated by an electromagnetic coil that surrounds the part and is supplied with alternating, or oscillating, electrical current from a power source. However, when a simple electromagnetic coil design and thorough heating of the part are desired, the induction heating process typically requires the use of a susceptor that encapsulates the part. Susceptors are not only electrically conductive, but also have a high thermal conductivity for a more efficient and thorough heating of the part. Therefore, manufacturing processes requiring localized heating, relatively quick heat-up and cool-down times, a more efficient use of power, or customized thermal properties that enable fabrication, benefit from induction heating processes that use susceptors.
Certain manufacturing processes require heating up to, but not beyond, a certain temperature. A select type of susceptor, often referred to as a xe2x80x9csmart susceptor,xe2x80x9d is constructed of a material, or materials, that generate heat efficiently until reaching a threshold, or Curie, temperature. As portions of the smart susceptor reach the Curie temperature, the magnetic permeability of those portions drops precipitously. The drop in magnetic permeability has two effects, it limits the generation of heat by those portions at the Curie temperature, and it shifts the magnetic flux to the lower temperature portions causing those portions below the Curie temperature to more quickly heat up to the Curie temperature.
Mechanical part manufacturing processes often require the controlled application of heat, such as when consolidating composite panels, or for metal forming processes such as brazing and superplastic forming. To this end, smart susceptors have been employed in combination with dies for mechanical forming such as the invention described in U.S. Pat. No. 5,728,309 to Matsen et al. commonly assigned and incorporated herein by reference. Matsen discloses an induction heating workcell 10 that includes a pair of ceramic dies 20, 22 mounted within a pair of strongbacks 24, 26. A pair of cavities 42, 44 defined by the dies hold respective ones of a pair of tool inserts 46, 48. A retort 60 is positioned between the tool inserts and includes a pair of susceptor sheets sandwiching a pair of metal, or composite, part panels. The tool inserts define a contoured forming surface 58 that has a shape corresponding to the desired shape of the upper and lower mold line surfaces of the completed part. An induction coil 35 is embedded into the dies and surrounds the cavities, tool inserts and the retort.
Suction pressure can be used to hold the susceptor halves to the dies when handling the dies before the start of the process. During the process, the retort is heated to forming or consolidation temperature by energizing the induction coil which generates an electromagnetic flux field. The flux field causes the susceptor plates to generate heat, while the dies and tool inserts have a relatively low magnetic permeability and therefore generate little heat. Internal tooling pressure is used to hold the susceptors against the dies during processing. This pressure is either supplied by sealing around the perimeter of the dies or using pressurized bladders. The application of heat and pressure is continued until the metal part panels are properly brazed, or formed, or the resin in the composite panels is properly distributed to form the completed part.
Advantageously, the susceptor may be custom tailored to the desired operating temperature by using different alloy materials such as cobalt/iron, nickel/iron, iron/silicon, or amorphous or crystalline magnetic alloys. Also, the susceptor can be designed to have several different operating temperatures by using multiple layers of different alloys that are tuned to different Curie temperatures.
Generally, the formation of parts having geometrically complex outer surfaces requires an equally complex mold surface. In an alternative design, the smart, susceptor may be constructed of a pair of metal sheets that are cold or hot formed so as to define an inner mold surface that corresponds to the desired part surface. Each formed metal sheet is positioned in the cavity of a respective one of the dies so that the dies support the formed metal sheets. During processing, the susceptor sheets generate heat and a vacuum pressure is applied to conform the part panels to the inner mold surface defined by the formed metal sheets. Unfortunately, the metal sheets used for the susceptor have forming limits which in turn tend to limit the complexity of the mold surface that can be defined by the metal sheets of the susceptor.
Therefore, it would be advantageous to have an induction heating device for the formation of geometrically complex parts. More particularly, it would be advantageous to have a system for forming a susceptor mold surface having a complex geometry, thereby allowing the manufacture of geometrically complex parts.
The present invention addresses the above needs and achieves other advantages by providing an induction heating device for manufacturing a part, including a geometrically complex part, by heating the part to a predetermined temperature. The induction heating device includes an induction coil connected to an electrical power supply for generating an electromagnetic flux field. A smart susceptor of the heating device is positioned in the electromagnetic flux field and includes a magnetically permeable material supported by a mesh structure. The magnetically permeable material generates heat in response to the flux field. The mesh structure provides support for the magnetically permeable material and closely conforms to the desired outer geometry of the part. The magnetically permeable material may be applied as a powder to the mesh using a hot spray gun, allowing tight conformance of the susceptor to the part geometry while avoiding forming limits of sheet metal susceptors.
In a first embodiment, the induction heating device includes an electrical power supply, an induction coil and a smart susceptor. The induction coil is operably connected to the electrical power supply and is capable of generating an electromagnetic flux field from the electrical power. The smart susceptor is positioned in the electromagnetic flux field. The smart susceptor includes a magnetically permeable material responsive to the electromagnetic flux to generate heat. Further, the smart susceptor includes a mesh structure supporting the magnetically permeable material. The mesh structure and the magnetically permeable material conform to at least a portion of the part so as to be capable of receiving the portion of the part and heating the portion of the part as the magnetically permeable material is heated.
The mesh structure may be constructed of wire capable of withstanding the heat generated by the electromagnetic flux. For instance, the mesh structure may be constructed of 0.02 inch thick stainless steel wire. Optionally, the mesh structure may also be constructed of magnetically permeable wire, such as a smart susceptor alloys to improve inductive heat generation.
The magnetically permeable material and mesh structure may define a cavity that conforms to all of the part. The mesh structure includes a pair of separable portions that are combinable to define the cavity. Preferably, the induction heating device further includes a die having a pair of die portions wherein each of the separable portions of the mesh structure are attached to a respective one of the die portions. In this manner, the die is configured to hold the separable portions of the mesh structure together so as to define the cavity.
In another embodiment, the present invention includes a method of manufacturing a smart susceptor for heating a part in response to the application of a magnetic flux field. The method of manufacturing includes providing a model having an outer geometry similar to that of the part. In one aspect, the model can be machined from richlite or aluminum. A mesh screen is draped over the part and conformed to the outer geometry of the model such as by tacking or gluing the mesh screen to the outer geometry of the model. Magnetically permeable material is deposited on the mesh screen and the mesh screen, with the permeable material, is released from the model.
The magnetically permeable material may be developed by thermally spraying a magnetically permeable powder onto the mesh screen. The screen and powder are bright annealed and sintered to consolidate the combination. Optionally, such bright annealing may occur in a hydrogen gas furnace. As another option, the consolidated powder and screen may be thermally sprayed on both sides with a nickel aluminide coating to protect against oxidation.
The present invention has several advantages. The use of the mesh screen and thermally sprayed, magnetically permeable powder allows the smart susceptor to be molded to geometrically complex parts, overcoming the forming limitations of previously used metal sheets. In particular, the flexible weaves of mesh closely drape to the geometrically complex parts while providing support for the magnetically permeable material to avoid failure during handling and processing. Increased complexity of part geometry allows a reduction in the number of parts used in assemblies, with a concomitant cost and weight reduction. Hydrogen gas furnace annealing reduces oxidation in the mesh and sprayed powder material, while the increased temperature increases the density of the material.